C220 refuses to idle sometimes

I drive a C220 1993 model and generally love the car. Recently it started cutting out when having to idle (for example when standing at a stop street
or even when at speed and just engaging the clutch with foot off the accelerator) but does not consistently die every time. This happens through all temperature ranges (from start-up to fully warmed up). The engine immediately starts again when the ignition is turned and then idles perfectly!
Any ideas?
Much oblidged Cornell.

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